It is known in the art of medical radiography to employ intensifying screens to reduce the X-ray dosage to the patient. Intensifying screens absorb the X-ray radiation and emit electromagnetic radiations which can be better absorbed by silver halide emulsion layers. Another approach to reduce the X-ray dosage to the patient is to coat two silver halide emulsion layers on the opposite sides of a support to form a duplitized radiographic element.
Accordingly, it is a common practice in medical radiography to use a radiographic assembly consisting of a duplitized radiographic element interposed between a pair of front and back screens.
A well known problem of this assembly relates to the cross-over phenomenon. Cross-over is due to light emitted from a screen which passes through the transparent film support and exposes the opposite silver halide emulsion layer. The result is a reduced sharpness of the resulting image due to light scattering caused by the support.
Many solutions have been suggested to reduce the cross-over problem, as disclosed for example, in Research Disclosure, August 1979, Item 18431, Section V, "Crossover Exposure Control". Research Disclosure is a publication of Kenneth Mason Publication Ltd., Erosworth, Hampshire PO10 7DD, United Kingdom.
The major part of the suggested solutions relates to the use of a filter substance absorbing the crossing light, as disclosed, for example, in Research Disclosure Vol. 122, June 1974, Item 12233, GB 1,426,277, GB 1,414,456, GB 1,477,638, GB 1,477,639, U.S. Pat. No. 3,849,658, U.S. Pat. Nos. 4,803,150, 4,997,750, and 4,994,355. The use of the above solution causes some other problems, such as, for example, efficiency reduction of the assembly, desensitization of the silver halide emulsion, worsening of the tint and/or tone of the developed radiographic element, longer developing time to eliminate the filter substance, and the like.
Other approaches relate to the use of reflecting underlayers or polarizing underlayers. Tabular silver halide grains are also known for their use to reduce cross-over, as disclosed in U.S. Pat. Nos. 4,425,425 and 4,425,426. These patents disclose that a reduction of cross-over is directly proportional with the increase of the aspect ratio, and the best results are obtained with tabular grains having an aspect ratio higher than 8:1.
In medical radiography another problem is related to the different X-ray absorption of the various parts of the body. For example, in chest radiography the heart area has an absorption ten times higher than the lung area. A similar effect occurs in the radiography of the stomach, where a contrast medium is used in order to enhance the image depictivity (the body part having no contrast medium being totally black), and of hands and legs, where bones have an X-ray absorption higher than that of soft tissues such as flesh and cartilage.
In these cases a radiographic element showing a low contrast is required for area of high X-ray absorption and a radiographic element showing a high contrast is required for area of low X-ray absorption. The resulting film is a compromise in an attempt to have sufficient optical density and sharpness for these different areas of the body. However, if the areas of low X-ray absorption are correctly exposed, the areas of high X-ray absorption are not correctly visible due to underexposure. On the other hand, if the areas of high X-ray absorption are correctly exposed, the other areas are totally black due to overexposure. Various methods have been suggested to solve this problem. One approach relates to the use of radiographic elements having two different emulsion layers coated on each side of the support. An example of this solution can be found in French patent 1,103,973, wherein the use of screens having a light emission ratio of from 1:1 to 1.5:1 (back screen:front screen) in combination with a radiographic element having coated thereon a high contrast back emulsion and a low contrast front emulsion is suggested. A combination of screens having a light emission ratio higher than 1.5:1 and radiographic elements having emulsion layers with the same gradation is also suggested. Other patents disclose the use of double coated radiographic elements having emulsion layers with different contrast or sensitivity. For example, DE 1,017,464 discloses a double coated radiographic element having coated thereon a first emulsion with high sensitivity and low contrast and a second emulsion with low sensitivity and high contrast, FR 885,707 discloses a double coated radiographic element having coated thereon a first high speed emulsion and a second high contrast emulsion, and FR 875,269 discloses a radiographic assembly comprising several radiographic films or papers, each having a different sensitivity and/or contrast relative to the others, in order to obtain separate and different images of the same object with a single exposure. Nothing in the above described patents suggested the use of the specific combination of the present invention to obtain a double-coated radiographic element showing a reduced cross-over, a super-rapid processability and optimal image quality. An approach similar to that of the above described French and German patents is disclosed in U.S. Pat. No. 4,994,355, claiming a double coated radiographic element having emulsion layers with different contrast, in U.S. Pat. No. 4,997,750, claiming a double coated radiographic element having emulsion layers with different sensitivity and in U.S. Pat. No. 5,021,327 claiming a radiographic assembly wherein the back screen and emulsion layer have a photicity at least twice that of the front screen and emulsion layer, the photicity being defined as the integrated product of screen emission and emulsion sensitivity. As discussed above, these patents require the use of a dye underlayer to reduce cross-over and moreover require a processing time of at least 90 seconds. Research Disclosure, December 1973, Vol. 116, Item 11620 discloses a radiographic element which shows different contrast when observed with or without a green filter, respectively. Finally, EP 126,644 disclosed a double coated radiographic element having silver halide emulsion layers with different contrast at different ranges of optical density.
A third more recent problem in medical radiography relates to the increased use of silver halide elements, which has led to a strong request for a reduction of processing times. If rapid processing of a film (i.e., a process shorter than 45 seconds) takes place, several problems can occur, such as an inadequate image density (i.e., insufficient sensitivity, contrast and maximum density), insufficient fixing, insufficient washing, and insufficient film drying. Insufficient fixing and washing of a film cause a progressive worsening of the image quality and modification of the silver tone. Moreover, the high temperature and the low gelatin content used for the reduction of the processing time cause the radiographic element to be marked by the pressure of the transporting roller. The use of hardening agents to fore-harden the silver halide radiographic element has been suggested, for example, in U.S. Pat. No. 4,414,304 but satisfactory results have not yet been obtained.
Accordingly, there is still the need of a radiographic assembly which solves the above mentioned problems.